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<a name="Output-Templates-and-Operand-Substitution"></a>
<h3 class="section">17.5 Output Templates and Operand Substitution</h3>
<a name="index-output-templates"></a>
<a name="index-operand-substitution"></a>

<a name="index-_0025-in-template"></a>
<a name="index-percent-sign"></a>
<p>The <em>output template</em> is a string which specifies how to output the
assembler code for an instruction pattern.  Most of the template is a
fixed string which is output literally.  The character &lsquo;<samp>%</samp>&rsquo; is used
to specify where to substitute an operand; it can also be used to
identify places where different variants of the assembler require
different syntax.
</p>
<p>In the simplest case, a &lsquo;<samp>%</samp>&rsquo; followed by a digit <var>n</var> says to output
operand <var>n</var> at that point in the string.
</p>
<p>&lsquo;<samp>%</samp>&rsquo; followed by a letter and a digit says to output an operand in an
alternate fashion.  Four letters have standard, built-in meanings described
below.  The machine description macro <code>PRINT_OPERAND</code> can define
additional letters with nonstandard meanings.
</p>
<p>&lsquo;<samp>%c<var>digit</var></samp>&rsquo; can be used to substitute an operand that is a
constant value without the syntax that normally indicates an immediate
operand.
</p>
<p>&lsquo;<samp>%n<var>digit</var></samp>&rsquo; is like &lsquo;<samp>%c<var>digit</var></samp>&rsquo; except that the value of
the constant is negated before printing.
</p>
<p>&lsquo;<samp>%a<var>digit</var></samp>&rsquo; can be used to substitute an operand as if it were a
memory reference, with the actual operand treated as the address.  This may
be useful when outputting a &ldquo;load address&rdquo; instruction, because often the
assembler syntax for such an instruction requires you to write the operand
as if it were a memory reference.
</p>
<p>&lsquo;<samp>%l<var>digit</var></samp>&rsquo; is used to substitute a <code>label_ref</code> into a jump
instruction.
</p>
<p>&lsquo;<samp>%=</samp>&rsquo; outputs a number which is unique to each instruction in the
entire compilation.  This is useful for making local labels to be
referred to more than once in a single template that generates multiple
assembler instructions.
</p>
<p>&lsquo;<samp>%</samp>&rsquo; followed by a punctuation character specifies a substitution that
does not use an operand.  Only one case is standard: &lsquo;<samp>%%</samp>&rsquo; outputs a
&lsquo;<samp>%</samp>&rsquo; into the assembler code.  Other nonstandard cases can be
defined in the <code>PRINT_OPERAND</code> macro.  You must also define
which punctuation characters are valid with the
<code>PRINT_OPERAND_PUNCT_VALID_P</code> macro.
</p>
<a name="index-_005c"></a>
<a name="index-backslash"></a>
<p>The template may generate multiple assembler instructions.  Write the text
for the instructions, with &lsquo;<samp>\;</samp>&rsquo; between them.
</p>
<a name="index-matching-operands"></a>
<p>When the RTL contains two operands which are required by constraint to match
each other, the output template must refer only to the lower-numbered operand.
Matching operands are not always identical, and the rest of the compiler
arranges to put the proper RTL expression for printing into the lower-numbered
operand.
</p>
<p>One use of nonstandard letters or punctuation following &lsquo;<samp>%</samp>&rsquo; is to
distinguish between different assembler languages for the same machine; for
example, Motorola syntax versus MIT syntax for the 68000.  Motorola syntax
requires periods in most opcode names, while MIT syntax does not.  For
example, the opcode &lsquo;<samp>movel</samp>&rsquo; in MIT syntax is &lsquo;<samp>move.l</samp>&rsquo; in Motorola
syntax.  The same file of patterns is used for both kinds of output syntax,
but the character sequence &lsquo;<samp>%.</samp>&rsquo; is used in each place where Motorola
syntax wants a period.  The <code>PRINT_OPERAND</code> macro for Motorola syntax
defines the sequence to output a period; the macro for MIT syntax defines
it to do nothing.
</p>
<a name="index-_0023-in-template"></a>
<p>As a special case, a template consisting of the single character <code>#</code>
instructs the compiler to first split the insn, and then output the
resulting instructions separately.  This helps eliminate redundancy in the
output templates.   If you have a <code>define_insn</code> that needs to emit
multiple assembler instructions, and there is a matching <code>define_split</code>
already defined, then you can simply use <code>#</code> as the output template
instead of writing an output template that emits the multiple assembler
instructions.
</p>
<p>Note that <code>#</code> only has an effect while generating assembly code;
it does not affect whether a split occurs earlier.  An associated
<code>define_split</code> must exist and it must be suitable for use after
register allocation.
</p>
<p>If the macro <code>ASSEMBLER_DIALECT</code> is defined, you can use construct
of the form &lsquo;<samp>{option0|option1|option2}</samp>&rsquo; in the templates.  These
describe multiple variants of assembler language syntax.
See <a href="Instruction-Output.html#Instruction-Output">Instruction Output</a>.
</p>
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